Viruses with an RNA genome are important human pathogens and exhibit distinct genetic and immunological properties from DNA viruses. Plant and invertebrate hosts produce virus-derived small interfering RNAs (siRNAs) of 21-24 nucleotides in length to guide specific clearance of RNA viruses by RNA interference (RNAi). However, it has been unclear if the conserved RNAi pathway has a similar antiviral role in mammals. New findings described here and in two recent papers in Science by the principal investigator provide evidence for the induction of antiviral RNAi and specific sterilizing immunity in mice by a mosquito-transmissible positive-strand RNA virus. Preliminary studies also indicate a role for the viral suppression of RNAi in mouse infection and for a known innate immune sensor of RNA viruses in the biogenesis of the predominantly 22- nucleotide viral siRNAs. Using the established mouse model, a complementary set of studies is proposed to characterize the function and mechanism of the mammalian RNAi- mediated antiviral immunity. Moreover, possible regulatory roles of interferon signaling and the interferon-stimulated innate immune sensors of RNA viruses in the new form of mammalian antiviral immunity will be examined. These studies will likely open up a new area of research on the antiviral immunity mechanisms in mammals.